Investigation of the chemical homogeneity of Cu80(Fe,Co)20 powders produced by mechanical milling

Cu80Fe14Co6 and Cu80Fe6Co14 alloys were elaborated by high-energy ball milling of pure Cu, Fe and Co powders for times up to 20 h. X-ray diffraction, 57Fe Moessbauer spectrometry and ZFC/FC magnetic measurements show that milling the powders for times up to 6 h leads to the progressive dissolution of Fe and Co atoms into Cu, forming a so-called 'solid solution'. For longer milling times, the structure of the as-milled powder does not change. However, the magnetic behaviour reveals that local chemical inhomogeneities exist. In some regions, the composition is similar to the nominal composition of the starting powder: Fe and Co atoms are dissolved in Cu and give rise either to a reentrant spin-glass behaviour in Cu80Fe14Co6 powders, or to a spin-glass behaviour in Cu80Fe6Co14 powders. Some other regions are enriched in Fe and Co and probably contain nanometer-sized α-Fe and α-Co precipitates that lead to a superparamagnetic behaviour at room temperature. The occurrence of such inhomogeneities have been confirmed by field ion microscopy observations and tomographic atom probe investigations, that clearly show that the Cu-Fe-Co solid solution which is formed after milling pure Cu, Fe and Co powders is chemically non-homogeneous